3 ms·
I intuited a probably-inaccurate trick with exponentially raising a small prime (e.g 7*7*7etc), doubling the multiple (*2), and subtracting 1 (to get an odd num
by eganist 5y ago
I intuited a probably-inaccurate trick with exponentially raising a small prime (e.g 7*7*7etc), doubling the multiple (*2), and subtracting 1 (to get an odd number).
That got me two substantially-different primes and narrowed my numbers a lot, which then became guesswork.
My guesses in the end:
-?---
?---!
?-??!
?!??!
!!!!!
- metalliqaz 5y agolol this would not help me at all by the way am I supposed to take some meaning from the italicized '7' or was that just a typo?
- ColinWright 5y agoHe probably had 7x7x7, but with asterisks instead of the "x"s.
- eganist 5y agocorrect, thanks for the heads up. Fixed
- madcaptenor 5y agoThe number you come up with in that way is at least guaranteed to not be divisible by 2 or 7. Obviously that doesn't mean it's prime but it definitely helps your odds.
- naniwaduni 5y agoAlso usefully, (2(a+1)^n-1) % a = (2a(a+1)^(n-1) + 2(a+1)^(n-1) - 1) % a = 1. So if you choose one of the 3k+1 primes, it's also guaranteed not divisible by 3.